1. Field of the Invention
The present invention relates to components associating in the same substrate vertical diffused-Type MOS power transistors (VDMOS) and logic circuits, and more specifically relates to the use of such components in circuits supplied by a battery, such as automobile circuits.
2. Discussion of the Related Art
FIG. 1 very schematically shows a portion of such a component. This component includes an N-type substrate generally formed of an N-type epitaxial layer 1 formed on an N.sup.+ -type substrate 2. A power transistor is formed in the right-hand portion and a logic well is formed in the left-hand portion.
The power transistor includes a set of identical cells connected to one another, such as cell 3. Each cell includes a P-type well 4, the central portion 5 of which is more heavily doped. An N-type ring 6 is formed in the upper portion of the well. The portion separating the external periphery of ring 6 from the external periphery of well 4 is coated with an isolated gate 8. N-type ring 6 as well as central portion 5 of the well are coated with a metallization 9. All gates 8 are connected to a gate terminal G and all metallizations 9 are connected to a source terminal S. The rear surface of the structure is coated with a drain metallization D. Thus, when a gate signal is applied, a current is likely to flow from terminal D to terminal S from N regions 1 and 2 to N regions 6, via a channel formed under the insulated gates. This structure is generally used so that the drain is biased to a positive potential with respect to the source.
Elements of logic circuits are formed in one or several wells 10. An elementary MOS transistor 11 having drain, source, and gate terminals g, d, and s has been shown in a well 10. This is only an example of a component that could be formed in a logic well.
The voltages on the several components formed in the logic well must be applied with respect to a reference. The simplest way to provide this reference, that is, to implement a ground connection, is illustrated in FIG. 1 and corresponds to the use of a well contacting area 12 formed on a region 13 of same type (P) as the well and more heavily doped. Contact 12 may be connected directly to the ground in simple embodiments. Generally, it should be understood that, for example, high voltages VDD are applied to some drains of the MOS transistors of the logic circuit which have their sources connected to contact 12 and to the ground.
FIG. 2 shows an example of an assembly of a component of the type shown in FIG. 1. The component is generally designated by block 20 surrounded with a frame in dotted lines. A reverse diode D1 is illustrated in parallel between the drain and the source of MOS power transistor T and corresponds to the junction between N-type substrate 1 and P-type region 5. Well 10 is represented by a block and it is assumed that it is connected to drain D of the MOS transistor via a diode D2 corresponding to the junction between substrate 1 and well 10.
In a very simple example of assembly, contact 12 of the well is grounded by a connection 21; and the source of the power transistor is connected to the ground via a load L, the switched supply of which is desired to be performed by the power transistor. A supply source such as a battery 23 is connected between the ground and drain terminal D of power transistor T. Thus, in normal operation, diodes D1 and D2 are reverse biased. According to its control, transistor T will be turned on or not and no current flows from the ground (contact 12) to the rear surface of the component (terminal D) due to the existence of reverse-biased diode D2.
Two incidents likely to occur in battery-powered circuits, and more specifically in automobile circuits, should however be considered.
The first incident corresponds to a biasing inversion of the battery. Diodes D1 and D2 are then forward biased. The current in diode D1 is limited by the presence of load L. Thus, the current will essentially flow through diode D2, as indicated by arrow 24. This current is likely to be destructive.
A second incident corresponds to a supply interruption, or battery disconnection, likely to occur when, due to vibrations or for any other reason, a lead wire of the battery breaks or operates intermittently. Then, if load L is inductive, the current will continue to flow therethrough according to the path designated by arrow 26. It should be noted that this current will necessarily exist, load L having to be considered as a current source. In the case of the simple assembly of FIG. 1, this current flow raises no specific problem. The problems result from the use of known protections against battery voltage inversions, as will be seen hereafter.
A first conventional solution to solve the battery inversion problem consists of inserting a diode, biased in a direction opposite to that of diode D2, in series with the well. The inserting of a diode may for example be performed in the way illustrated in FIG. 1, by adding an N.sup.+ -type region 15, grounding this region 15 by a connection 17, and suppressing the connection to ground 21. Various solutions have been provided to optimize the operation of this diode, and to have a well referenced to the ground when the circuit is in a normal operation state. Reference will especially be made to U.S. Pat. No. 5,099,302 (Antoine Pavlin) relating to an active diode which is incorporated herein by reference. There still remains the problem that, in the case of a battery disconnection, current 26 will have to cross an avalanching diode and will dissipate a high power therein, which can cause a destruction of the component, unless a diode having a large surface is provided, which unduly increases the cost of the component.
A second conventional solution to the battery inversion problem consists of placing in series in connection 21 a resistor, a terminal of which will form the ground connection. But a new dilemma, difficult to solve, arises. Indeed, in normal operation, the resistance must be as low as possible to limit the voltage drop thereacross caused by the consumption of the elements of the logic circuit. Conversely, to solve the problems linked to the battery inversion case, this resistance must be as high as possible to limit the current flowing through the well.